One conclusion I've drawn over the last couple of weeks is that the canned profiles that ship with the 4800 are really quite good and hold their own beside custom profiles. I have noticed an improvement in prints with the latter, but to me it isn't huge (or at least hasn't been to date) - if I had to quantify the improvement , maybe 5% overall.

(Which makes sense in a way - why would Epson ship a product like the 4800 with profiles for their own papers which weren't very good?)

The improvements in prints when moving to custom from canned profiles on the 2100 prints was easily more noticeable - although still not huge. Related, now that I'm back into production mode I'm able to compare a wider range of prints produced on the 2100 using custom profiles to prints on the 4800 using canned and custom profiles. In all cases the 4800 prints have had the edge - I'm noticing more things to like about it's color performance, especially how clean the lighter tones are compared to the profiled 2100 prints.

Laying out all my 4800 test prints to date on Glossy, Semigloss, Pictorico Velvety, Archival Matte, Elegance and Photo Rag, some using canned profiles, some custom - they are all very similar. There are shifts/variation print to print but nothing dramatic, aside from things mentioned previously e.g. the blacks on Elegance.

At least when you use custom profiles you know you are theoretically getting the best out of the combination of your paper/your printer combination - the money spent is probably worth it for the peace of mind alone.